It is conventionally known that chlorinated vinyl polymers can be rendered electroconductive by being subjected to a dehydrochlorination to produce many long conjugated multiple bond chains therein. Specific methods of achieving this are as follows.
(a) Thermal Method
When a chlorinated vinyl polymer is heated, hydrogen chloride is vigorously released. To take polyvinyl chloride as an example, it melts at 170.degree. C. and above and hydrogen chloride is vigorously released at 190.degree. C. and above to form conjugated multiple bonds (see "Kagaku Binran Oyohen (Chemical Handbook--Applications)", p. 786, complied by The Chemical Society of Japan, published by Maruzen, 1980).
(b) Chemical Method
According to Synth. met., Vol. 17, 143 (1987), chlorinated polyvinyl chloride is submerged in a strong base solution of DUB (1,8-diazabicyclo[5.4.0]-7-undecene) and a dehydrochlorination is permitted to proceed ionically to form an electroconductive material.
(c) Photochemical Method
Light emitted from a mercury lamp and other sources of light in the ultraviolet wavelength region dissociate carbon-to-chlorine bonds in a chlorinated vinyl polymer and the resulting chlorine radicals extract adjacent hydrogen atoms to cause a dehydrochlorination.
According to a report in J. Polym. Sci., Polym. Letters, Vol. 25, 5 (1987), a chlorinated polyvinyl chloride was irradiated with a medium-pressure mercury lamp to produce chlorinated polyacetylene photochemically, followed by irradiation with an argon ion laser at 488.1 nm to prepare an electroconductive material.
As described above, various methods have been known for performing a dehydrochlorination to impart electroconductivity to chlorinated vinyl polymers. However, they have their own problems as summarized below. First, it is difficult to produce a uniformly thin film by the thermal method (a). In order to initiate a dehydrochlorination, the polymer must be heated to its softening point or a higher temperature in which case the polymer is subjected to deformation or other unavoidable deterioration occurs. In the chemical method (b), a strong base is used to initiate a dehydrochlorination and, as a result, a salt that is deleterious to electroconductivity is produced in the thin film formed but this salt is unremovable.
In the photochemical method (c), light emitted from a mercury lamp and other sources of light in the ultraviolet wavelength region initiate a dehydrochlorination in the chlorinated vinyl polymer but, at the same time, prolonged exposure to such light causes photocrosslinking and cleavage of the backbone chain. Further, exposure to light in the ultraviolet region in an oxygen-containing atmosphere will cause oxidation of the chlorinated vinyl polymer and subsequent cleavage of its backbone chain, resulting in the formation of carbonyl groups at the points of cleavage. Such photocrosslinking and cleavage of the backbone chain are side reactions that are unfavorable for the purpose of extending the conjugated multiple bond chains which are necessary to impart electroconductivity to the chlorinated vinyl polymer of interest.
J. Polym. Soc., Polym. Letters, Vol. 25, 5 (1987) mentioned under (c) "Photochemical Method" and PROC. SPIE-INT. Soc., OPT. ENG. Vol. 1022, 112-117 (1988) mentioned in the International Search Report describe techniques similar to the idea of the present invention but they differ from the present invention in terms of the wavelength of irradiation and the pulse duration. According to those prior art reports, conductivity is developed upon irradiation with continuous visible light (488.1 nm) from an argon ion laser. The reports also state the failure to develop conductivity by irradiation with a nitrogen laser emitting pulsed light at a wavelength of 337 nm.